In a variety of clinical settings, it is important to measure certain chemical characteristics of plasma from whole-blood samples. For example, it is commonly important to measure the analytes, extracellular hemoglobin, bilirubin, and lipid particles in plasma. These settings range from a routine visit of a patient to a physician's office, an emergency room, or monitoring of a hospitalized patient, for example. Numerous techniques and apparatus are commonly used for measuring chemical characteristics of body fluids in clinical settings. Measurement of an analyte in a body fluid sample may be accomplished by numerous methods such as spectroscopic determination and refractometry, for example.
Some techniques for analyzing body fluid are complex and may involve numerous steps such as centrifugation to prepare a fluid sample for measurement. For example, techniques for measuring analyte content in the plasma portion of a blood sample may involve preliminary steps such as centrifugation of whole blood to separate blood cells from the plasma portion. These preliminary steps add time, complexity and cost to previously known techniques for measuring analyte content in a body fluid.
Previous techniques for measuring the total protein content of fluid sample have generally involved measuring optical absorption after addition of a reagent. By contrast, total protein content can also be measured without reagents by quantifying the refractive index of the fluid. The protein content of a substance can be determined based on its refractive index because there is a well-known direct relationship between the refractive index and the protein content.
Refractometry is commonly performed to determine the total protein content of blood plasma by measuring the refractive index of a plasma sample. Previous techniques for using refractometry to measure total protein content in blood involve preliminary steps such as centrifugation to separate cells from the blood. Refractometry is then performed on a sample of the cell free plasma. It has previously been assumed that pure plasma is needed for measuring total protein in blood using refractometry because the presence of blood cells introduces a large light scattering potential which can disrupt the refractometry measurements. Without compensation or optical assess to cell depleted plasma, scattering of light by cells in whole blood significantly reduces the accuracy of a measurement of refractive index.